A problem affecting such overhead cranes is local deflection or subsidence of the runway or its supports. If the bridge frame of the crane is rigid, and the trucks directly support the frame, any resulting lack of alignment of the track can result in gross inequality of the loads transmitted through the different trucks. This results in increased stresses being applied to the runway, those trucks whose loading is increased, and the frame. In extreme cases, even quite small runway deflections can result in the load applied to one or even two of the trucks being reduced to zero. Various approaches to this problem have been adopted. One solution is to render the bridge frame sufficiently flexible to accomodate deflections and subsidences of the runway, typically either by forming the frame of two spaced but linked members or by placing a flexible link in the periphery of the frame. Such techniques permit distortions in the frame which would otherwise give rise to very high torsional stresses. Disadvantages of these solutions are that a distortion of the bridge frame will also result in distortion of the secondary runway provided on the frame for the trolley, which must therefore itself be designed to accomodate such distortion. Furthermore, the tracking of such a frame as it moves along the main runway is inferior to that of a rigid frame, with the result that the frame may tend to become misaligned on the runway. If a rigid frame is retained, problems in maintaining alignment of the trolley runway are eliminated, and tracking problems are reduced. On the other hand, the transfer of loads between the trucks supporting the frame on the main runway will result in very high torsional stresses being applied to the frame. Thus the designers of the crane and the runway must consider the interaction between the stiffness characteristics of the frame, and the probable degree of subsidence and deflection likely to occur in the runway. It is of course often difficult to estimate the likely degree of subsidence of a runway in advance; although it should in theory be possible to design foundations for the runway structure which in most cases will be free of subsidence, in practice an unpredictable amount of subsidence often tends to occur.